Miniature igniter

ABSTRACT

The invention relates to a novel miniature igniter which comprises a pair of electrically conductive spaced apart members each having a complementary recess at one end, an electrically insulative plate intermediate the members also having a recess complementary to the recesses of the electrically conductive members, all of said recesses forming a cavity for receipt of charge, a bridgewire spanning the plate to electrically interconnect the conductive members, charge material in the recess over the bridgewire, and connector pins in electrical connection with the conductive members for electrically integrating the igniter in an electrical circuit.

United States Patent Craig et a1.

[ 1 Sept. 23, 1975 of Albuquerque. N. Mex.

[73] Assignee: The United States of America as represented by the United States Eneregy Research and Development Administration, Washington, DC.

[22] Filed: July 30, 1974 [21] Appl. No.: 493,007

[52] U.S. C1 102/28 R [51] Int. Cl. F42B 3/12 [58] Field of Search 102/28 R [56] References Cited UNITED STATES PATENTS 2,942.546 6/1960 Liebhafsky 102/28 2,981,186 4/1961 Stresan 102/28 3,135.200 6/1964 Jackson 102/28 3,198,117 8/1965 Purdy et all l. 102/28 Moses et a1 102/28 Dahl 102/28 Primary ExamlnerVerlin R. Pendegrass Attorney, Agent, or FirmDean E. Carlson; Ignacio Resendez; Dudley W. King [57] ABSTRACT The invention relates to a novel miniature igniter which comprises a pair of electrically conductive spaced apart members each having a complementary recess at one end, an electrically insulative plate intermediate the members also having a recess complementary to the recesses of the electrically conductive members, all of said recesses forming a cavity for receipt of charge. a hridgewire spanning the plate to electrically interconnect the conductive members. charge material in the recess over the bridgewire. and connector pins in electrical connection with the conductive members for electrically integrating the igniter in an electrical circuit.

7 Claims, 5 Drawing Figures US Patent Sept. 23,1975 3,906,858

MINIATURE IGNITER BACKGROUND OF INVENTION The invention relates to miniature electrical igniters or squibs.

Squibs or igniters for igniting explosive charges in ballistic missiles, thermal batteries, gas generators, and the like, are known. Generally, these comprise lead pins or wires disposed in holes in a suitable insulator and having a bridgewire connecting end portions of the pins at some point adjacent the explosive charge. The insulators may often be made of glass or ceramic materials.

It is desirable in some applications to minimize the size of the igniters to achieve a desired purpose. Problems may be encountered in attempting to minimize or reduce the size of these igniters due to various inherent constraints. For example, because of the limited ability of some prior art igniters to absorb pressure, it is gener ally not feasible to compress, within the igniter body, the explosive charge by application of high pressures in order to compact the explosive material into a small volume and achieve intimate, compressed and direct contact of the explosive charge with the bridgewire.

It is also difficult to reduce the pin spacing to control bridgewire length by drilling or otherwise forming holes in the glass or ceramic such that the ends of the lead pins or lead wires adjacent the explosive charge are closer than about 0.025 inches. Drilling may create concealed cracks which extend from one hole to adjacent holes beneath the surface, which cracks may subsequently create electrical shorts and other problems in operation of the igniter or squib.

Bridgewire pin Spacing is important because as the fast pulse firing energy requirement is reduced in order to meet specifications for miniaturized systems and the bridge resistance is specified, the bridge volume must be reduced. It is difficult to comply with these specifications using prior art technology because of the limitations hereinabove described.

SUMMARY OF INVENTION In view of the above limitations, it is an object of this invention to provide a miniature igniter suitable for use as the ignition source in thermal batteries, explosive cartridges, gas generators, and the like.

It is a further object of this invention to provide a novel igniter wherein bridgewire pin spacing may be as low as 0.010 inches.

It is a further object of this invention to provide an igniter having a recess wherein the explosive charge may be satisfactorily compressed to pressures in excess of 90,000 pounds per square inch (psi).

It is a further object of this invention to provide an igniter having massive, electrically conductive members to which the bridgewire is engaged to provide maximum heat dissipation capability.

It is a further object of this invention to provide an igniter having a one ohm bridge which reliably initiates upon delivery of 100,000 ergs in less than three microseconds without the use of extremely sensitive primary explosives,

Various other objects and advantages will become apparent from the following description of this invention and the most novel features will be pointed out with particularity hereinafter in connection with the appended claims. It is understood that various changes in the details, materials and process steps which are herein described and illustrated to better explain the nature of the invention may be made by those skilled in the art without departing from the scope of this invention.

The invention comprises, in brief, an igniter having a pair of electrically conductive, spaced apart members each having a complementary recess at one end, an electrically insulating plate intermediate the members also having a recess complementary with the recesses of the conductive members, the members recesses and the plate recess forming a cavity receiving a charge, an electrically conductive bridgewire disposed in the cavity spanning the insulating plate and having opposite end portions electrically connected to the electrically conductive members, a closure disc disposed over the cavity, and connector pins in electrical connection with and projecting from the conductive members for connecting to an electrical circuit.

DESCRIPTION OF DRAWING FIG. 1 illustrates, in a cutaway, partially crosssectional and partially perspective view, an embodiment of this invention;

FIG. 2 is a cross-sectional representation of a portion of the embodiment shown in FIG. 1;

FIGS. 3 and 4 are cross-sectional views of portions of alternate embodiments of this invention; and

FIG. 5 is a top view of an embodiment of an igniter of this invention, without the charge or end cover, illustrating an alternate mode for positioning the bridgewire.

DETAILED DESCRIPTION The igniter 10 of this invention may be of generally cylindrical configuration II as illustrated in FIG. I. The elements of this igniter I0. a portion of which is shown in greater detail in FIG. 2, may include a pair of electrically conductive members 16a, 16b each having a recessed portion 18a, 18b at one end, the members having adjacent large planar surfaces 190, I911 being spaced or separated from each other by a relatively thin, electrically insulativc plate 22 having oppositely disposed planar faces 20a, 20b disposed intermediate and in direct contact with the electrically conductive members 16a, 16b. Electrically conductive members may be of any desirable shape, such as rectangular or of generally semicircular, elongated configuration. Electrically insulative plate 22 likewise has a recessed portion 21 at one end thereof which matches or com plements adjoining or contiguous recessed portions 180, I812 in electrically conductive members l6a, [6b and which forms therewith a cup, chamber or cavity 26 which receives or houses charge 34. Walls forming Cav ity 26 may define any shape such as a generally semicircular or cylindrical shape illustrated in the drawings. Cavity 26 may preferably be in coaxial alignment with the longitudinal axis of the igniter assembly 10.

Electrically conductive members 16a, 16b may be made of such materials as nickel and its alloys or clad materials such as copper-nickel and the like, and are preferably made of materials that are good electrical conductors such as nickel-iron and niekel-iron-cobalt alloys. Electrically insulative plate 22 may be made of any suitable ceramic material having good metallizingbrazing capabilities as known in the art. To obtain maximum heat dissipation, it may be made of alumina which contains at least 94.0 weight percent aluminum oxide (A1 Ceramic plate 22 may be bonded or joined to members 16a, l6b through any suitable joining process such as a molybdenum-manganese brazing process. In this brazing process, a thin layer of molybdenum-manganese coating may be applied to the alumina ceramic plate 22 to a thickness of from about 0.002 to about 0.005 inches. An electrodeposition coating may thereafter be applied over the molybdenum-manganese coating. The electrodeposition coating may be to a thickness of about 0.0004 inches using a suitable material such as nickel. The ceramic may then be sintered in a protective atmosphere such as a hydrogen atmosphere at a temperature of about 1475C to improve adherence of the metal to the ceramic and reduce the tendency of the ceramic to oxidize. The electroplated surface may then be soldered or brazed onto the adjoining planar surfaces 19a, l9b of the electrically conductive members 160, l6b using a suitable braze material such as 72 weight percent copper and 28 weight percent silver yielding a brazed connection or joint 27 which joins plate 22 and surfaces 190, l9b of members l6a, 16b. Processes for applying the molybdenum-manganese coating, the nickel electroplating and sintering, and the soldering or brazing are generally known in the art. Prior art igniters had small diameter lead pin holes in the ceramic insulators which limited the use of these processes to deposit coatings within pin hole walls. The present igniter eliminates these pin holes and thereby does away with this coating limitation.

Disposed within cavity 26 is a bridgewire 28 which spans plate 22 and is suitably joined or connected to electrically conductive members 16a, l6b at opposite ends of the bridgewire 28. The mode ofjoining bridgewire 28 to the conductive members may be such as by resistance spot welds 30a, 30b. Also disposed within cavity 26 is the charge or pellet 34 which may comprise one or more types of charges. For example, as illustrated in FIG. 2, disposed within cavity 26 is an igniter charge 36 which is ignition sensitive to the electrical input stimuli and an output charge 38 which is an ignition source for use in thermal batteries or the like. The igniter charge 36 is disposed in recess 26 in direct contact with bridgewire 28, electrically conductive members 16a, 16b, and plate 22. The igniter charge may be compressed in place by applying a ram (not shown) at exposed surface 40 of igniter charge 36 at a suitable pressure such as from about 2,500 to about 90,000 psi and preferably at about 15,000 psi. It is un derstood that the amount of charge that may be contained in cavity 26 is related to the compression force employed.

It may be desirable to dispose intermediate igniter charge 36 and output charge 38 a perforate dielectric layer 42, such as a mica layer, of a thickness of from about 0.002 to about 0.005 inches having one or more openings or apertures 44 to permit heat from igniter charge 36 to pass to output charge 38 and ignite same while reducing the potential of electrical shorting through conductive by-products generating from the combustion of igniter charge 36 and output charge 38. Output charge 38 is in physical contact with igniter Charge 36 through port, aperture or opening 44 of dielectric layer 42. Depending upon the type and chemical composition of charges employed, mica layer 42 may not be required, or may be positioned intermediate conductive members and ceramic plate 22 and igniter charge 36, as shown in FIG. 3, in which output charge 38 is in direct contact with igniter charge 36. Mica layer 41 shown in FIG. 3 may have an aperture to allow layer 41 to fit over the bridgewire 28 and thus be contiguous to the electrically conductive members and igniter charge 36. An alternate method for effecting the same purpose as mica layer 42 recited hereinabove, i.e., reduce the potential of electrical shorting through conductive by-produets generating from the combustion of igniter charge 36 and output charge 38, may comprise depositing an insulative coating 45 by suitable vapor deposition techniques on the walls and bottom of cavity 26 as shown in FIG. 4. Coating 45 may preferably be aluminum oxide at a thickness of from about 5,000 angstroms to about 50,000 angstroms and preferably about 10,000 angstroms. A suitable maskant may be employed to mask the bridgewire area to prevent deposition onto the bridgewire, or, alternatively the bridgewire area may also be coated and the coating subsequently abraded.

Output charge 38 may be disposed within cavity 26 to be contiguous with either igniter charge 36 (FIG. 3) or dielectric layer 42. A ram may be placed within the periphery of cavity 26 and output charge 38 may be compressed at a suitable pressure of from about 2,500 to about 90,000 psi, and preferably at about 15,000 psi, in the same manner as igniter charge 36 was com pressed. It may be desirable not to compress igniter charge 36 and output charge 38 individually but to compress them simultaneously. A suitable dielectric barrier, layer or disc 43 may be placed over output charge 38 to isolate charge 38 from the closure mem bet or cover 46 material which seals cavity 26. A suit able dielectric may be such as a polyimide resin.

A rupturable closure disc, pad, or cover 46 is disposed over igniter l0 sealing cavity 26 adjacent output charge 38 and against disc 43, said closure member extending over the ends or nonrecessed portions 48a, 48b of electrically conductive members 1611, l6!) respectively as well as the nonrecessed portions of the plate 22. Disc 46 may be of any desirable configuration such as a flat disc, or may have a thickened central portion 47. Disc 46 is made of a suitable dielectric material,

such as synthetic rubber, which may be joined to the igniter members by a suitable adhesive. The sealing process seals the cavity contents from an external atmosphere. Dielectric disc 43 may be placed in cavity 26 contiguous to and overlying output charge 38 to prevent any moisture from touching output charge 38 in the sealing process. Disc 43 may likewise be from 0.002 to about 0.005 inches thick.

Members 16a, lob also have, or are in electrical connection with, suitable means for engaging igniter 10 into an electrical circuit. For example, electrical connector pins 52 may be disposed within appropriate bores or cavities 56 at any part of the electrically conductive members 160, 16b and may then be suitably joined to these members by suitable joining procedures or processes, such as brazing. to result in a brazed joint or interconnection 60. Various other joining or connector means for interconnecting igniter 10 into an electrical circuit may be used. Electrical connector pins 52 may be made of the same material as electri cally conductive members 16a, 161) or of other appropriate electrically conductive materials.

It may be desirable to apply a suitable dielectric coating 65 such as a fluorocarbon material or the like, which may extend over exterior walls of figniter igniter except those parts which are required to be exposed for electrical contact such as electrical connector pins 52. The igniter end which is covered by cover or closure disc 46 may or may not be coated. Coating 65 provides electrical insulation as well as corrosion protection and the like properties. Cover 46 not only aids in retaining the explosive charge 34 within the cavity 26, but also prevents moisture from entering explosive charge 34.

Various materials such as nickel-chromium and gold alloys which are capable of being resistance spot welded or ultrasonically welded to electrically conductive members 16a, 16b may be used for bridgewire 28. Bridgewires having a composition of 80 nickel-20 chro mium have been successfully employed. These bridgewires have a resistance of about one ohm and a diameter of about 0.0012 inches. The length of the bridgewire and diameter are selected from considerations of the no-fire, all-fire power encountered by the bridgewire, the bridge resistance specified and the like. The igniter of this invention may receive several different bridgewire lengths by positioning the bridgewires so as to diagonally span the electrically insulative ceramic plate as illustrated by bridgewire 28a in cavity 26 in FIG. 5.

The igniter of this invention provides attainment of maximum no-fire electrical current standoff and firing sensitivity for a given pyrotechnic or explosive material which may be used. igniter design offers maximum heat sinking to the bridgewire as well as permits consolidation or compression pressure of in excess of 90,000 psi directly against the bridgewire and its substrate. The laminate construction of this invention may advantageously use electrically insulative alumina ceramic plates having a thickness which may be as small as 0.010 inch. Spacing across the electrically insulative ceramic plate as noted above is an important factor because as the fast pulse Firing energy requirement is re duced and the bridge resistance is specified at a low level, the bridge volume (length) may be reduced.

This invention permits the use of pyrotechnic or secondary explosive charges such as used in exploding bridge wire applications in lieu of the much more sensitive and potentially hazardous primary explosive charges. Examples of pyrotechnic materials that have been initiated with a one ohm resistance, 0.0012 inch diameter bridgewires include: boron/calcium chromate, zirconium/potassium perchlorate, potassium perchlorate/potassium hexacyanocobaltate, boron/potassium perchlorate/titanium, boron/potassium perchlorate/iron oxide, and boron/potassium perchlorate/- calcium chromate.

Nofire sensitivity tests, defined as the maximum energy level which may be applied to an electricalexplosive device for a given period without igniting or degrading the surrounding explosive material. were conducted on an igniter of this invention having electrically conductive members made of nickel-iron alloys, connector pins made of the same alloy, electrically insulative ceramic member made of about 94% alumina, bridgewire made of nickel-chromium and being of one ohm resistance and 0.0012 inch diameter. Results using boron/calcium chromate as the igniter charge resulted in a mean no-fire current level at 5 minutes of 1.21 am peres with a sigma of 0.0285 ampere. Sigma refers to a measure of the deviation of a number from measurements for the mean.

A higher no-fire current level may be obtained using clad material, or other suitable components instead of nickel-iron alloys because of the higher thermal conductivity, Bridgewire volume must be controlled to allow the pyrotechnic igniter to fire on microseconds duration/low energy pulse. lgniters of this invention having a one ohm bridge have been made with a suffi ciently small volume to insure reliable initiation of the selected powder or ignition charge with 100,000 ergs delivered in microseconds, and in general will initiate in less than three microseconds. Assuming a normal distribution, the estimated energy level at which 99.9% will fire is 70,000 ergs. This fast response is achieved by this invention while still maintaining a higher no-fire standoff as discussed hereinabove. lgniters of this in vention wherein the electrically insulative ceramic plate length is about 0.180 inches and wherein the ignition charge is 20% boron/% calcium chromate at a weight of 4.5 milligrams and wherein the output charge is 40% titanium/60% potassium perchlorate at a weight of 9.5 milligrams have been successfully employed.

What is claimed is:

1. An igniter comprising a pair of electrically conductive spaced apart members each having a complemen tary recess extending inwardly from one end thereof, an electrically insulative ceramic plate intermediate said members maintaining them spaced apart having a recess extending inwardly from one end thereof corrc sponding with said member recesses and forming there with an open-ended charge cavity, an electrically conductive bridgewire disposed at the bottom of said cavity spanning said insulative ceramic plate and having opposite end portions electrically connected with said spaced apart members, an igniter charge in said cavity in contact with said bridgewire, and a closure disc extending over said cavity and sealed to said electrically conductive members and said insulativc plate.

2. The igniter of claim 1 including connector pins in electrical connection with and projecting from said conductive members at end portions opposite said cav ity.

3. The igniter of claim 1 wherein said ceramic member thickness intermediate said electrically conductive members is about 0.010 inch, said ceramic is alumina, and said electrically conductive members are selected from the group consisting of nickel, iron, and alloys thereof.

4. The igniter of claim 1 including an output charge disposed over said igniter charge in said cavity.

5. The igniter of claim 4 including a perforate dielec tric member disposed intermediate said igniter charge and said output charge, heat from said igniter charge passing through said perforation in said perforate dielectric member to ignite said output charge.

6. The igniter of claim 5 wherein said bridgewire is a one ohm resistance bridgewire at a thickness of about 0.0012 inch. said bridgewire is made of nickelchromium alloy, said insulating ceramic plate is 0.020 inches thick, said igniter charge is boron/calcium chromate, said output charge is titanium/potassium perchlorate, and said igniter has a mean no-firc current level at 5 minutes of about 1.2] amperes.

7. An igniter comprising a pair of electrically conductive, spaced apart semicircular members each having a complementary semicircular recess extending inwardly rom one end thereof, each of said members having a ilanar surface extending to said semicircular recess; an :lectrically insulative, about 94% alumina ceramic ilate of thickness of at least 0.010 inches intermediate aid electrically conductive semicircular members at a liameter thereof maintaining them spaced apart and laving a recess extending inwardly from one end hereof corresponding with said member recesses and arming therewith an open ended charge cavity, said eramic plate having a pair of oppositely disposed plaiar faces for bonding to said conductive member plaiar surfaces; brazed bonds intermediate said insulative llate planar faces and said conductive members planar urfaces; an electrically conductive bridgcwire disiosed at the bottom of said cavity spanning said ceamic plate and having opposite end portions electrially connected with said spaced apart members; a ompresscd igniter charge in said cavity in contact with said bridgewire, said igniter charge compressed in said cavity at from about 2,500 psi to about 90,000 psi, a compressed output charge disposed over said igniter charge in said cavity, said output charge compressed in said cavity at from about 2,500 psi to about 90,000 psi; a perforate dielectric layer disposed intermediate said igniter charge and said output charge, heat from said igniter charge after ignition passing through said perforations in said perforate dielectric layer to ignite said output charge,a closure disc extending over said cavity and sealed to said electrically conductive members and said insulative plate, a dielectric disc disposed in said cavity intermediate said output charge and said closure disc to separate said output charge and said closure disc material, and connector pins in electrical connection with and projecting from said conductive members at end portions opposite said cavity. 

1. An igniter comprising a pair of electrically conductive spaced apart members each having a complementary recess extending inwardly from one end thereof, an electrically insulative ceramic plate intermediate said members maintaining them spaced apart having a recess extending inwardly from one end thereof corresponding with said member recesses and forming therewith an open-ended charge cavity, an electrically conductive bridgewire disposed at the bottom of said cavity spanning said insulative ceramic plate and having opposite end portions electrically connected with said spaced apart members, an igniter charge in said cavity in contact with said bridgewire, and a closure disc extending over said cavity and sealed to said electrically conductive members and said insulative plate.
 2. The igniter of claim 1 including connector pins in electrical connection with and projecting from said conductive members at end portions opposite said cavity.
 3. The igniter of claim 1 wherein said ceramic member thickness intermediate said electrically conductive members is about 0.010 inch, said ceramic is alumina, and said electrically conductive members are selected from the group consisting of nickel, iron, and alloys thereof.
 4. The igniter of claim 1 including an output charge disposed over said igniter charge in said cavity.
 5. The igniter of claim 4 including a perforate dielectric member disposed intermediate said igniter charge and said output charge, heat from said igniter charge passing through said perforation in said perforate dielectric member to ignite said output charge.
 6. The igniter of claim 5 wherein said bridgewire is a one ohm resistance bridgewire at a thickness of about 0.0012 inch, said bridgewire is made of nickel-chromium alloy, said insulating ceramic plate is 0.020 inches thick, said igniter charge is boron/calcium chromate, said output charge is titanium/potassium perchlorate, and said igniter has a mean no-fire current level at 5 minutes of about 1.21 amperes.
 7. An igniter comprising a pair of electrically conductive, spaced apart semicircular members each having a complementary semicircular recess extending inwardly from one end thereof, each of said members having a planar surface extending to said semicircular recess; an electrically insulative, about 94% alumina ceramic plate of thickness of at least 0.010 inches intermediate said electrically conductive semicircular members at a diameter thereof maintaining them spaced apart and having a recess extending inwardly from one end thereof corresponding with said member recesses and forming therewith an open ended charge cavity, said ceramic plate having a pair of oppositely disposed planar faces for bonding to said conductive member planar surfaces; brazed bonds intermediate said insulative plate planar faces and said conductive members planar surfaces; an electrically conductive bridgewire disposed at the bottom of said cavity spanning said ceramic plate and having opposite end portions electrically connected with said spaced apart members; a compressed igniter charge in said cavity in contact with said bridgewire, said igniter charge compressed in said cavity at from about 2,500 psi to about 90,000 psi, a compressed output charge disposed over said igniter charge in said cavity, said output charge compressed in said cavity at from about 2,500 psi to about 90,000 psi; a perforate dielectric layer disposed intermediate said igniter charge and said output charge, heat from said igniter charge after ignition passing through said perforations in said perforate dielectric layer to ignite said output charge,a closure disc extending over said cavity and sealed to said electrically conductive members and said insulative plate, a dielectric disc disposed in said cavity intermediate said output charge and said closure disc to separate said output charge and said closure disc material, and conneCtor pins in electrical connection with and projecting from said conductive members at end portions opposite said cavity. 